Some test, laboratory and diagnostic equipment, such as oscilloscopes, digital multimeters, electrocardiograms and the like, receive electrical signals from devices or subjects under test and display or provide information about the signals. Other equipment, such as waveform generators, generate electrical signals for injection into circuits, subjects, etc. (Signal receiving and signal generating equipment is hereinafter collectively referred to as “test equipment” or “test instrument.”) In test equipment, sometimes a signal must be attenuated by a known amount. For example, an oscilloscope can be used to display on a screen a waveform of a signal. The screen is of finite size; however, the oscilloscope can be used to display waveforms of signals having small amplitudes, as well as signals having large amplitudes.
To accommodate a wide range of signal amplitudes, the oscilloscope includes a set of attenuators, typically resistors. Each attenuator in the set can attenuate the signal by a different amount. Thus, a desired amount of attenuation can be achieved by switching a combination of one or more of the attenuators into a signal path of the instrument. The attenuators are used to “scale” the input signal, such as by dividing the amplitude of the signal by 1, 2 or 5 and/or by a power of 10 (i.e., by 10, 100, 1000, etc.) Signal generators, such as function generators, waveform generators, digital signal generators and the like, also employ switched attenuators, so they can produce signals having desired amplitudes.
Prior art test equipment includes manually operated mechanical switches or electromechanical relays in their chassis to switch attenuators into or out of signal paths. An electromechanical relay includes an electromagnet that, when energized, actuates a spring-loaded set of contacts to make and/or break an electrical circuit. However, these switches and relays are large and, therefore, introduce large amounts of parasitic capacitance into the signal path. This capacitance can distort the signal, thereby leading to inaccurate renderings of waveforms, etc. For example, the parasitic capacitance and resistance of the attenuators create a low-pass filter, due to the resistance-capacitance (RC) time constant of the combination of components. This filter limits the bandwidth of the test equipment.
Switches and electromechanical relays also have other drawbacks. For example, such switches and relays require their contacts to be cleaned frequently. Electromechanical relays draw a significant amount of power. In addition, mechanical switches and electromechanical relays can not be operated at high switching speeds.
It would, therefore, be desirable to switch various attenuators into or out of a signal path without the parasitic capacitance, bulk, slow speed or high power consumption characteristic of the prior art.